What is passivity of iron?

							
The inertness exhibited by metals under conditions when chemical activity is to be expected is called chemical passivity. The fllowing are the common properties of iron.
(a) It evolves hydrogen gas, when made to react with dilute HCL or dilute H2SO4.
(b) It precipitates silver from silver  nitrate solution and copper from copper sulphate solution.
But if a piece of iron is first dipped in concentrated nitric acid for sometime and then made to react with the above regents, neither hydrogen is evolved nor silver ot copper are precipitated. Thus, iron by treatment with concentrated nitric acid has lost its usual properties or it has been rendered inert or passive. Such behaviour is not only shown by iron but also by many other emtals like Cr, CO, Ni, Al etc. This phenomenon si known as passivity and the chemical substances, which bring passivity, are called passivators.
Other oxidising agents can render iron passive like chromic acid, KMnO4, concentrated H2SO4 etc. The passivity of the iron is believed to be due to formation of an extremely thin film of oxide on the surface of iron.